Conventional hydraulic automotive brake systems utilize a source of hydraulic pressure to actuate a piston to bias a brake shoe having a friction material surface into contact with a brake drum in a drum brake system or to bias a piston to move a brake pad containing friction material into contact with a vertical face of a rotor in a disk brake system. Because of the out-of-roundness inherent in brake drums and of the lateral run out inherent in rotors, the friction element alternately engages low and high spots on the brake drum or rotor. Because of the relative incompressibility of the brake fluid the pressure in the brake system experiences a sharp increase when the friction element engages a high spot on the drum or rotor and experiences a relative pressure decrease when the friction element rubs a low spot on a drum or rotor. These pressure fluctuations which occur in the brake fluid cause pressure waves, surges, spikes and harmonics to propagate through the hydraulic system.
Typically, a pressure wave would move from a wheel cylinder or disk brake piston to the brake master cylinder and thereafter be reflected back from the master cylinder to the wheel cylinder or brake caliper piston. Very high momentary braking pressures occur within the hydraulic system when the reflected pressure waves, surges, spikes and harmonics moving toward the brake cylinder or piston add to clamping force already exerted thereon.
In common hydraulic automotive brake systems one or more wheels of the vehicle may lock or skid during severe braking applications while the other wheels are rotating which may cause the vehicle operator to experience a loss of control. It may be demonstrated that wheel lockup occurs because a friction element becomes "stuck" on a so called high spot on a disk brake drum or rotor. This wheel lockup occurs because the high spot initiates a high pressure wave into the hydraulic system which moves from a wheel cylinder or brake piston towards the master cylinder and reflects back through the brake line and adds to the clamping force already exerted on the shoe or caliper. Consequently, it has been found that the addition of a small accumulator to the hydraulic system will absorb pressure surges to maintain a constant fluid pressure at each actuator piston and thereby reduce the tendency of a friction element to prematurely become "stuck" on a brake drum or rotor high spot.
One type of accumulator which has been added to an automobile brake system to reduce pressure or surges may be seen by referring to U.S. Pat. No. 3,430,660 to Mitton. The Mitton utilizes a hollow resilient bulb contained within a housing having concave side walls. A space exists between the outside surface of the bulb and the inner wall of the housing. Additionally, a central cavity within the bulb is in fluid communication with the vehicle hydraulic fluid. Consequently, when pressure peaks or surges are encountered within the system the side walls of the bulb expand outwardly to absorb the pressure surges or transients and maintain a constant hydraulic pressure at all vehicle wheels.
In U.S. Pat. No. 4,571,009 to Jones, the inventor claims an advantage in having the space between the outer walls of the resilient bulb and the inner walls of the housing pre-pressurized with air. This pre-pressurization occurs by making a substantial length of the bulb with a larger diameter than the opening of the housing into which the bulb is inserted. Consequently, after the bulb has been inserted into the housing the air in the space between the bulb and the housing is pressurized.
Although, the accumulators described in the aforementioned patents function to reduce pressure surges within a vehicle brake system, they suffer a number of disadvantages. To begin with, the devices are not universally applicable to all vehicle hydraulic systems. As an example, the hydraulic brake system for a motorcycle contains a small volume of fluid compared to the volume of brake fluid in an automobile or truck or bus hydraulic system. Thus, a different accumulator must be manufactured for each of these systems. Additionally, the accumulators do not discriminate between vehicles which are heavy or light in weight, vehicles which carry a large percentage of their weight on the front of the vehicle as opposed to the rear of the vehicle and high performance vehicles such as racing cars which have different braking requirements than conventional passenger cars. Accumulators must be custom made for each application. Also, in the past it has been difficult to obtain an accumulator which will provide the optimum amount of brake pedal pressure or the optimum amount of brake travel required during the braking process. Additionally, prior accumulators have been unable to provide a desired feel of the brake pedal or feedback from the brake pedal to the operator during the brake process in vehicle brake system with accumulator. In some vehicle applications the brake pedal may feel spongy whereas in other vehicle applications the brake pedal may feel very stiff to an operator after an accumulator has been installed. Also, in some vehicle applications a relatively large amount of brake pedal travel occurs whereas in other applications very little brake travel occurs as a result of the addition of an accumulator within the vehicle hydraulic system. Furthermore, neither of the aforementioned prior art accumulator devices provides a simple means of adjusting the device to obtain a desired or proper brake pedal feel or amount of travel.
It has been found that where an accumulator device of the type described in the aforementioned Mitton patent has been added to a vehicle hydraulic brake system the accumulator device must be initialized prior to obtaining proper operation thereof. Such an initialization requires that a vehicle operator make several severe braking applications or panic stops subsequent to installation of the device. This initialization must occur each time the integrity of the hydraulic system is disturbed.
As mentioned previously, the accumulator device described in the aforementioned Jones patent pre-pressurizes the space surrounding the resilient bulb within the housing by trapping a volume of air within that space during installation of the resilient element within the housing. It has been found that as the under hood temperature of a vehicle changes the feel of a brake pedal during the braking process also changes in vehicles equipped with the Jones type of accumulator. It has been discovered that such a change results from the fact that the trapped air within the space between the resilient bulb and the metal housing changes pressure with temperature because of impurities including moisture within the air. This moisture may also cause a degradation of the accumulator device and other brake system components.
From the above, it may be seen to be desirable to provide a pressure control device for a vehicle hydraulic brake system which may be adjusted readily to provide an optimum brake pedal feel and travel amount for any desired vehicle, which does not require an initialization process subsequent to installation which may be utilized in vehicles having large or small volume hydraulic systems, which will accommodate light or heavy vehicles or vehicles which have a greater percentage of vehicle weight on one end of the vehicle or the other, which will work satisfactorily on high performance vehicles and which maintains a more constant brake pedal feel and amount of travel without regard to vehicle under hood temperature.